ABSTRACT Illusory position shifts induced by motion suggest that motion processing can interfere with perceived position. This may be because accurate position representation is lost during successive visual processing steps. We found that complex motion patterns, which can only be extracted at a global level by pooling and segmenting local motion signals and integrating over time, can influence perceived position. We used motion-defined Gabor patterns containing motion-defined boundaries, which themselves moved over time. This 'motion-defined motion' induced position biases of up to 0.5 degrees , much larger than has been found with luminance-defined motion. The size of the shift correlated with how detectable the motion-defined motion direction was, suggesting that the amount of bias increased with the magnitude of this complex directional signal. However, positional shifts did occur even when participants were not aware of the direction of the motion-defined motion. The size of the perceptual position shift was greatly reduced when the position judgement was made relative to the location of a static luminance-defined square, but not eliminated. These results suggest that motion-induced position shifts are a result of general mechanisms matching dynamic object properties with spatial location.

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Objects in motion appear shifted in space. For global motion stimuli we can ask whether the shift depends on the local or global motion. We constructed arrays of randomly oriented Gaussian enveloped drifting sine gratings (dynamic Gabors) whose speed was set such that the normal component of motion was consistent with a single global velocity. The array appears shifted in space in the direction of the global motion. The size of the shift is the same as for arrays of uniformly oriented dynamic Gabors that are moving in the same direction at the same global speed. Arrays made up of vertically oriented gratings whose speeds were set to the horizontal component of the random array elements were shifted less far. This shows that motion-induced position shifts of coherently moving surface patches are generated after the completion of the global motion computation.

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The perceived position of stationary objects can appear shifted in space due to the presence of motion in another part of the visual field (motion drag). We investigated this phenomenon with global motion Gabor arrays. These arrays consist of randomly oriented Gabors (Gaussian windowed sinusoidal luminance modulations) whose speed is set such that the normal component of the individual Gabor's motion is consistent with a single 2D global velocity. Global motion arrays were shown to alter the perceived position of nearby stationary objects. The size of this shift was the same as that induced by arrays of Gabors uniformly oriented in the direction of global motion and drifting at the global motion speed. Both types of array were found to be robust to large changes in array density and exhibited the same time course of effect. The motion drag induced by the global motion arrays was consistent with the estimated 2D global velocity, rather than by the component of the local velocities in the global motion direction. This suggests that the motion signal that induces motion drag originates at or after a stage at which local motion signals have been integrated to produce a global motion estimate.

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Visual figures may be distinguished based on elementary motion or higher-order non-Fourier features, and flies track both. The canonical elementary motion detector, a compact computation for Fourier motion direction and amplitude, can also encode higher-order signals provided elaborate preprocessing. However, the way in which a fly tracks a moving figure containing both elementary and higher-order signals has not been investigated. Using a novel white noise approach, we demonstrate that (1) the composite response to an object containing both elementary motion (EM) and uncorrelated higher-order figure motion (FM) reflects the linear superposition of each component; (2) the EM-driven component is velocity-dependent, whereas the FM component is driven by retinal position; (3) retinotopic variation in EM and FM responses are different from one another; (4) the FM subsystem superimposes saccadic turns upon smooth pursuit; and (5) the two systems in combination are necessary and sufficient to predict the full range of figure tracking behaviors, including those that generate no EM cues at all. This analysis requires an extension of the model that fly motion vision is based on simple elementary motion detectors and provides a novel method to characterize the subsystems responsible for the pursuit of visual figures.

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Biol. Lett. (2009) 5, 270–273doi:10.1098/rsbl.2008.0622Published online 6 January 2009NeurobiologyThe movement of motion-defined contours can biasperceived positionSzonya Durant*and Johannes M. ZankerDepartment of Psychology, Royal Holloway, University of London,Egham TW20 0EX, UK*Author for correspondence (szonya.durant@rhul.ac.uk).Illusory position shifts induced by motionsuggest that motion processing can interferewith perceived position. This may be becauseaccurate position representation is lost duringsuccessive visual processing steps. We found thatcomplex motion patterns, which can only beextracted at a global level by pooling andsegmenting local motion signals and integratingover time, can influence perceived position. Weused motion-defined Gabor patterns containingmotion-defined boundaries, which themselvesmoved over time. This ‘motion-defined motion’induced position biases of up to 0.58, muchlarger than has been found with luminance-defined motion. The size of the shift correlatedwith how detectable the motion-defined motiondirection was, suggesting that the amount ofbias increased with the magnitude of thiscomplex directional signal. However, positionalshifts did occur even when participants were notaware of the direction of the motion-definedmotion. The size of the perceptual position shiftwas greatly reduced when the position judge-ment was made relative to the location of astatic luminance-defined square, but not elimi-nated. These results suggest that motion-induced position shifts are a result of generalmechanisms matching dynamic object proper-ties with spatial location.Keywords: Gabor pattern; shift;spatial representation1. INTRODUCTIONThe processing of motion-defined boundaries canprovide depth cues in optic flow and help to breakcamouflage. It involves the integration of local motionsignals over large areas in order to extract globalchanges in the motion patterns. It is necessary at thesame time to maintain a localized spatial signalassociated with such boundaries. Past work has inves-tigated our ability to localize such contours (Burr et al.2006; Durant & Zanker 2008). It has been shown thatluminance-based motion extraction processes interactwith the perceived position associated with areasof uniform motion (Ramachandran & Anstis 1990;DeValois & DeValois 1991). DeValois & DeValois(1991) compared the position of drifting Gaborpatterns (sinusoidal luminance patterns bounded byGaussian envelopes) contained within stationaryenvelopes with each other and found that perceivedposition of the envelopes of the patterns was biased inthe direction of motion. This effect shows a spatialand temporal frequency tuning. Bressler & Whitney(2005) used similar stimuli with contrast-definedmotion, and also found a position bias, although withdifferent spatial and temporal frequency tunings. It hasoften been suggested that second-order position cod-ing and motion processing are carried out differently(e.g. Kingdom et al. 1995; Sutter et al. 1995; Lu &Sperling 2001). Pavan & Mather (2008) compared thetwo different types of motion and suggested thatthe separate motion mechanisms feed into separateposition assignment mechanisms, with no interaction.We ask whether perceived position can be shiftedby motion, which in itself is defined by motion. Tosee this motion, extraction stages are needed, whichdiffer from those for contrast-defined motion. Severallayers of motion processing, larger spatial integrationareas and longer integration times than luminance-defined motion (Zanker 1992), as well as arguablyattentional tracking (Lu & Sperling 2001), arerequired. Maruya et al. (2008) found no effect of themotion of motion-defined contours on spatialposition. Here, we investigate with a stimulus analo-gous to the original (DeValois & DeValois 1991)stimulus and compare the positions of two motion-defined Gabor patterns containing drifting carrierpatterns (figure 1).2. MATERIAL AND METHODS(a) StimulusTwo motion-defined patterns (figure 1) were presented horizontallyon either side of a central fixation target (at 38 eccentricity),contours oriented horizontally with their carriers drifting in verti-cally opposite directions. Three thousand (five dots per 18 square)randomly positioned moving black dots (1 pixel size Z0.058; lifelimited to three frames) were presented on a bright grey back-ground (73 cd2mK1). The motion axis of the dots was eitherhorizontal (parallel to the contours) or vertical (orthogonal). Thevelocity of the dots (maximum 3 pixels per frame Z4.5 deg sK1)was determined by a Gabor pattern (figure 1). Sub-pixel positionaccuracy was calculated and rounded to the nearest pixel. Speedsbelow 0.3 pixels per frame were set to a random velocity between0.3 and 3 pixels per frame; carrier speed, 1.7 pixels per frame(2.55 deg sK1); presentation time, 60 framesZ2 s (30 Hz refreshrate). A random starting phase was chosen independently for eachpatch. Gabor patches were 48 full width at half height. The dotswere contained within a square area of 24.58 width. In experiment3, the right-hand pattern was a 4.58 width black square outline,0.58 thick. The experiment was approved by the Royal HollowayPsychology Department ethics committee.(b) ProcedureThe 2AFC method of constants was used. Seven offsets wereshown equally spaced between the left being higher or lower by 38.The position of both patterns was also shifted vertically by 0.758randomly on each trial, so the fixation point could not be used as aspatial reference. Participants indicated, using mouse buttons,which patch was higher. Eight responses were collected at eachoffset and a psychometric curve fitted with a logistic function,yielding the point of subjective alignment (PSA). The individualshift of a pattern was the average of the PSA offsets for oppositedirections (divided by 2 when two moving patterns werecompared). Four measurements were made for each condition. Thefour different conditions (left/right up and orthogonal/parallel) wereinterleaved during a block. For the judgement of the direction ofmotion task, the offsets were randomized and each of the conditionswas shown 10 times. The participants indicated which patterncontained upward motion of the motion-defined contours, and thenumber of correct responses was recorded.Electronic supplementary material is available at http://dx.doi.org/10.1098/rsbl.2008.0622 or via http://journals.royalsociety.org.Received 24 October 2008Accepted 5 December 2008270This journal is q 2009 The Royal Society on May 14, 2012 rsbl.royalsocietypublishing.org Downloaded from

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3. RESULTSWe began by finding conditions leading to sizeablepositional shifts and testing how this related to thevisibility of the motion of the motion-defined con-tours. We considered motion orthogonal and parallelto the contours. In experiment 1, we found that fora low spatial frequency of 0.1 cycles degK1of motionmodulation (containing only one or two motion-defined contours at any time), there was a significantshift in perceived position. This was greater fororthogonal than parallel motion, the maximum shiftbeing approximately 0.48–0.58 (figure 2a).In experiment 2, we tested with Gabors of a spatialfrequency of 0.7 cycles degK1(nine contours presentin one frame) and the same contour speed as inexperiment 1. We found a significant perceptual shiftin position of the envelopes of the Gabor patches,although the shift was reduced on the whole, and forthree participants confined to dots moving orthog-onally to the contours (figure 2b). This showed thatthe position shift is not limited to the particularstimulus conditions in experiment 1, although a widerparameter space remains to be explored.We founda significantcorrelation(rZ0.7,p!0.05) between the perceived position bias forexperiments 1 and 2 and the detectability of thecorresponding ‘motion of the motion’ direction(figure 2c): confirming that the more visible the motionof the contours, the larger the positional shift. However,we also found some points where performance on thedirection judgement is at chance levels, while thereremains a significant position shift, suggesting that it isnot necessary to consciously perceive the motion ofthe contours to perceive a shift in position.In experiment 3, we tested whether the perceivedshift found with the low spatial frequency motioncontours would be reduced by reducing the positionaluncertainty of the spatial reference. We compared theperceived shift relative to a hard luminance-edgedsquare as was done by Maruya et al. (2008)—who didnot find a position shift for this motion of the motion.We found that the shift was reduced and the patternwas less consistent across participants. In general,the difference between the two types of motionwas reduced; however, again, for all subjects apartfrom participant AS there is still a significant percep-tual position shift (figure 2d).4. DISCUSSIONWe found that the motion of motion-defined con-tours can induce illusory shifts in position. Thiseffect is particularly strong when the motion isorthogonal to the contours, when there are only afew contours visible and when the positions of twopatches are being compared with each other. Theperceived position shift of up to 0.58 is much largerthan the shifts found in the luminance domain ofaround maximum 10 minarc at similar eccentricities(DeValois & DeValois 1991). This suggests thathigh-levelmechanismscomplex motion signals can bias perceived position,and that the magnitude of the shift could be relatedto the coarse-grained representation of location atthese stages.The luminance-definedinduced shift increases with eccentricity (DeValois &DeValois 1991), which may reflect the fact of lowerspatial resolution in the periphery. The coarse-grained representation associated with global motioncould lead to increased positional uncertainty forthe location of these stimuli. The slopes of thepsychometric functions show a just noticeabledifference of around 15 minarc,than the accurate spatial representation in the lumi-nance domain with a resolution of approximately2 minarc at similar eccentricities (DeValois &DeValois 1991).We also observed a shift (albeit much reducedand less consistent) using a first-order (luminance-defined) stimulus as reference, suggesting that thetwo spatial position assignment mechanisms are notcompletely independent of each other (figure 2d).The decrease in perceived shift with the higher spatialfrequency motion carrier reflects that motion con-tours are less easily perceived (Watson & Eckert1994). The size of the perceived shift increased withthe saliency of motion of motion-defined contours,involvedin extractingmotion-much higher(a)(b)envelopecarrierFigure 1. Illustration of the stimulus in the ‘horizontal’(contour) ‘parallel’ (motion) condition. Black and whitearrows illustrate opposite directions of motion of the blackdots. (a) The large dashed arrow shows the overall directionof the motion contours. (b) A cross section of the velocityprofile at the line vertically through the middle of (a). Thephase change of the carrier is the ‘motion of the motion’.Motion-defined contours and position biasesS. Durant & J. M. Zanker 271Biol. Lett. (2009) on May 14, 2012 rsbl.royalsocietypublishing.orgDownloaded from

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suggesting that it was related to the magnitude of thishigher order motion signal. Importantly, however, anawareness of the motion-defined motion directionwas not necessary to produce a significant shift inperceived position (figure 2c), as was also found withluminance (Whitney 2006) and contrast-definedmotion (Harp et al. 2007).It is not clear why there is a larger positionalshift for the orthogonal motion-defined boundaries.At these low spatial frequencies, no difference insensitivity between the two conditions was foundpreviously for static contours (Nakayama et al. 1985).On average over the trials, there is no greater upwardor downward motion signal in this stimulus (as the(a)(c)(b)(d)size of positionshift (deg.)0.50.40.30.20.10.60size of positionshift (deg.)0.50.40.30.20.10.60size of position shift (deg.)0.50.40.30.20.10.600.50.40.30.20.10.600.50.40.30.20.10.60–0.1orthogonal parallelorthogonal parallel orthogonalparallel orthogonalparallelorthogonalparallel orthogonal parallel orthogonalparallel orthogonalparallel0.60.50.40.30.20.1–0.2–0.101.00.90.80.7proportion of correct responses0.60.50.40 0.1 0.2size of position shift (deg.)0.3 0.4 0.50.6(i) (ii) (iii)(iv)(i)(ii) (iii)(iv)(i)(ii) (iii)(iv)Figure 2. Measurements of the perceived position shift. Averages and SEM error bars calculated from four measurements ofthe psychometric function. (a) The size of the perceived position shift for the two types of contours at a low spatial frequency(0.1 cycles degK1; grey bars, horizontal; black bars, vertical). (i) Participant SD, (ii) participant AM, (iii) participant AS and(iv) participant FH. For participant SD, two arrangements (horizontally either side of fixation or the whole screen rotated tovertically either side) of the Gabor patterns were measured. (b) Position shift for the two types of contours at a high spatialfrequency (0.7 cycles degK1). (i) Participant SD, (ii) participant AM, (iii) participant AS and (iv) participant FH.(c) Position shift from experiments 1 and 2 plotted against the corresponding detection rate of the ‘motion of the motion’direction. The points are circled where performance on the direction judgement is at chance, but there is still a significantposition shift (open circles, orthogonal; filled circles, parallel). (d) The position of the motion-defined Gabor envelopecompared with the square frame. (i) Participant SD, (ii) participant AM, (iii) participant AS and (iv) participant FH.272S. Durant & J. M. ZankerMotion-defined contours and position biasesBiol. Lett. (2009) on May 14, 2012rsbl.royalsocietypublishing.org Downloaded from